1. Field of the Invention
This invention relates generally to light amplifier tubes and is concerned more particularly with an image intensifier tube having a highly efficient output screen.
2. Discussion of the Prior Art
Generally an image intensifier tube comprises a tubular envelope having a photocathode disposed adjacent a radiation transmissive faceplate at an input end of the envelope, and an imaging screen supported adjacent a light transmissive faceplate at the output end of the envelope. The photocathode usually includes a layer of photoemissive material disposed to receive a radiational image of an external object and to emit an equivalent electron image in the direction of the imaging screen. In operation, the imaging screen generally is maintained at a relatively high positive potential with respect to the photocathode for establishing an electron accelerating field therebetween. As a result, the electron image emitted from the photocatode is electrostatically focused onto the imaging screen with sufficient kinetic energy to produce a corresponding visual image which may be viewed through the output faceplate of the tube.
The imaging screen generally comprises a layer of phosphor powder material, such as silver activated zinc cadmium sulfide, for example, which may be deposited on a transparent substrate by conventional means, such as settling or centrifuging, for examples. The phosphor powder layer fluoresces locally in accordance with the spatial distribution of energy in the electron image, thereby producing a corresponding visual image. However, it has been found that due to the particulate nature of the deposited powder material numerous voids are developed throughout the imaging screen layer. The voids do not contribute to production of the visual image, and cause scattering of the visible light produced by the adjacent phosphor material. As a result, the deposited layer of phosphor powder material produces a visual image having low contrast characteristics and is very inefficient in converting incident electron energy into visible photon energy.
Therefore, it is advantageous and desirable to provide an image intensifier tube with an output imaging screen made of high packing density material for producing a visual image having improved contrast characteristics, and efficient conversion of incident electron energy into photons of visible light.